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[Towertalk] Common-mode choke at 430 Mhz

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Subject: [Towertalk] Common-mode choke at 430 Mhz
From: (Tom Rauch)
Date: Wed, 17 Jul 2002 21:31:00 -0400
> 1 Will a common-mode choke work on 430 MHz?

Certainly it would. You could make one from semi-rigid small diameter 
coaxial line air wound.

There are other choices that would work also. The best is probably a 
1/4 wl stub balun, where the coaxial feed has a parallel same-
diameter as the coax wire. Connect the parallel wire to the same 
terminal as the center of the feedline, and ground it to the shield 
1/4 wl away from the feedpoint. Like a small open wire line.

Another is a sleeve balun. Use a large diameter thin rigid metal tube 
over the coax. Attach it to the feedline shield 1/4 wl away from the 
antenna terminals, and let the end at the terminals float.

With ALL of these baluns, you must watch the dielectric outside the 
coax. It slows the Vp of the wave, limits balun impedance due to 
losses, and can make your calculated lengths incorrect. Best to use 
bare semirigid or rigid coax in all of them, with mostly air spacing.

Powdered iron or special low mu ferrites as a string of beads might 
also work if power levels are low. You'd have to be careful selecting 
materials, and be careful how you ground the cable past the beads. 
The cable needs a GOOD RF ground at 430 MHz just past the beads or 
any other common mode choke.

I don't know how you calculated required impedance, because to 
accurately know that you would have to know the COMMON mode impedance 
of the system, which is totally different than and not related to the 
differential mode impedance across the antenna terminals. If you 
added a choke when the cables common mode impedance was high, or 
capacitive, it could increase common mode currents. Adding a balun 
with X times the feedline Zo isn't always a good idea.
> 2   Will the capacity coupling between turns of the balun at 430 MHz
> make the choke ineffective and if so would winding the balun by
> spacing the turns the diameters of the RG-58 help?

It is a common but untrue myth that shunt capacitance always hurts a 

It will not hurt at all, as long as the self-resonant frequency does 
not go far below the operating frequency. As a matter of fact, shunt 
capacitance can actually INCREASE the balun impedance if it causes 
the balun to approach or pass self-resonance at the operating 

Take for example a 100 ohm inductor with a 1000 ohm shunt 
capacitance. The end reactance is 111 ohms. A shunt capacitor always 
increases the inductance unless the shunting capacitance is HALF the 
reactance of the inductance or less. For example, a 100 choke with a 
50 ohm shunt reactance has a combined reactance of 100ohms, but the 
sign is capacitive. It takes less than 50 ohms of parallel shunt C to 
reduce the combined reactance (assuming low loss components).
> 3   How would one check the common-mode currents on the yagi models?

Add a length of wire and attach it to one feed terminal. Look at the 
current in that wire as you vary the length. By the way, you can 
add a load that represents the balun common mode impedance and watch 
current as you vary length. You will see that many times adding a 
balun makes things worse.

The best choke for UHF is probably a stub-type balun Larry, because 
it does not depend on feedline common mode impedance being low or 
inductive to work. The next best choke would be one that is 
intentionally parallel resonant, because it would again work with 
almost any feedline common mode impedance.

73, Tom W8JI 

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